The human cells are constantly subjected to genotoxic stresses from both exogenous and endogenous DNA damage agents. Defective response to the DNA damage has detrimental consequence for human health. The goal of this R21 project is to identify the genetic defect in a newly defined radiation sensitivity disease, Dubowitz Syndrome. A skin fibroblast cell line, designated CoDa cells, has been established from a Dubowitz Syndrome patient who displayed severe skin injury after a short course of radiation therapy of cancer. Preliminary studies suggested that the CoDa cells are hypersensitivity to radiation due to a defective DNA repair system, and strongly indicated that there is a previously un-characterized genetic defect in CoDa cells. Combining the results of these preliminary studies and the unique clinical appearance of the Dubowitz Syndrome, it was hypothesized that a new genetic defect is responsible for the radiation sensitivity in CoDa cells. Built on a functional complementation strategy, this R21 project will use a retrovirus based approach to perform a non-biased screen of a cDNA library, to identify the wild type gene whose expression in CoDa can correct the radiation sensitivity. The identification of this gene will provide a novel entry point to decipher additional mechanisms of DNA damage response, and has the potential to identify a new molecular marker for environmental risk assessment and a new target for DNA damage sensitization in cancer therapy. Considering the hypersensitivity to radiation of the CoDa cells, this study will have strong impact on radiation biology and DNA repair fields.